Coronary ccllateral flow was monitored over 5 hours following occlusion in untreated dogs, and in dogs treated for one hour (beginning 10 minutes post-occlusion) with nitroglycerin and methoxamine. In untreated dogs, collateral flow was unchanged over 5 hours following occlusion. Treatment with nitroglycerin and methoxamine significantly increased collateral flow, and the increase in flow was maintained over the 5 hours studied, despite the fact that treatment was discontinued at 70 minutes post-occlusion.